This invention relates to phase modulators, particularly for use in microwave circuit and communications applications.
This invention finds use in a number of microwave modulator applications requiring direct, full 360 degree linear phase control of a carrier signal. The modulator can be used for analog phase modulation (PM) or digital phase modulation, including both phase shift keying (PSK) and continuous phase modulation (CPM) techniques. Another application of this modulator is for indirect frequency modulation (FM) or indirect frequency shift keying (FSK) modulation. Modulators employing any of these techniques can be made frequency agile. Other useful applications of the invention include phase synchronization of antenna and oscillator arrays, phased array antenna beam steering, and ultra-small carrier frequency translations.
A high performance, cost effective microwave phase modulator solution is very attractive in the design of microwave circuits and communication systems. One method of reducing the cost and complexity of the modulator is to exploit the advantages of directly modulating the microwave carrier signal. Direct modulation at the transmit frequency removes the requirement for multiple stages of intermediate frequency (IF) circuitry, upconversion, and filtering circuitry, which makes an elegant modulator architecture at microwave frequency feasible.
This invention is a simple and elegant hardware architecture for direct phase modulation, which is suitable for implementation at microwave frequencies. The modulator is based on a novel circuit architecture for achieving wideband linear phase modulation of a microwave carrier signal over the full 360 degree range with low phase error.
This invention presents an alternative hardware architecture for direct phase modulation at microwave frequencies. The modulator is based on a novel circuit architecture for wideband linear phase modulation of a microwave carrier signal over the full 360 degree range. With this method, a conditioned baseband modulating signal is injected into a highly linear phase shifter, operating at a subharmonic of the desired output frequency. A nonlinear circuit is then used to perform frequency and instantaneous phase multiplication, thus expanding the linear phase modulation range to greater than 360 degrees at the desired output frequency. With special conditioning of the baseband modulating signal, the phase modulator can be made frequency agile in ultra-small frequency steps, for any of the modulation techniques mentioned, without requiring a stable, frequency agile reference signal or frequency synthesizer.
This phase expansion and its application to direct linear phase modulation is the key to the invention. Use of the nonlinearity relaxes the linearity requirements of the phase shifter to a fraction of the desired 360 degree range, which makes realization of a highly linear fractional range phase shifter feasible. Using this nonlinear circuit technique, the modulator can achieve the high degree of linearity over the full 360 degree phase modulation range required for high performance phase modulation applications with low phase error. The modulator can readily be extended to very high microwave frequencies and have a large and highly linear phase modulation range, simply by increasing the order of the nonlinearity. The benefits of this new implementation include:
a. Simplified, cost effective hardware architecture, requiring only a single stage fractional range phase shifter, and very few or no active microwave devices.
b. Use of a frequency stable subharmonic reference signal, which is easier to obtain than the desired higher frequency carrier signal.
c. Use of a linear phase shifter over a fraction of the full 360 degree range resulting in higher phase linearity and low phase error.
d. Phase expansion in the linear modulation range by a factor equal to the order of the frequency/phase multiplier nonlinearity, which increases the linear modulation range in excess of 360 degrees. The phase error can be reduced without limit as the order of the nonlinearity is increased.
e. Extension to arbitrarily high microwave frequencies by increasing the order of the nonlinearity.
f. An effective increase in output bandwidth due to the frequency/phase multiplier by the order of the nonlinearity when compared with the subharmonically modulated signal.
g. Injection of conditioned baseband modulating signals to produce analog phase modulation (PM) or digital phase modulation, including both phase shift keying (PSK) and continuous phase modulation (CPM) techniques, as well as indirect frequency modulation (FM) or indirect frequency shift keying (FSK) modulation.
h. Frequency agility in ultra-small frequency steps, for any of the modulation techniques mentioned above, without requiring a stable, frequency agile reference signal or synthesizer.
i. Ability to provide rapid, frequency stable, ultra-small frequency or phase translations.
There is therefore provided in accordance with one aspect of the invention, a phase modulation device comprising a voltage controlled phase shifter having an input port for injection of a first signal having a first frequency and a first frequency/phase multiplier operatively connected to the voltage controlled phase shifter for receiving the first signal from the voltage controlled phase shifter and for translating the first signal to a second signal having a second frequency, the second frequency being higher than the first frequency.
In a further aspect of the invention, the phase modulation device comprises a voltage controlled phase shifter having an input port for injection of a first signal having a first instantaneous phase and an output port with a second signal having a second instantaneous phase and a first frequency/phase multiplier operatively connected to the voltage controlled phase shifter for receiving the second signal from the voltage controlled phase shifter and for expanding the instantaneous phase of the second signal to a value greater than 360 degrees.
In a further aspect of the invention, there is provided a method of phase modulating a signal comprising the steps of phase shifting a first signal at a first frequency and translating the first frequency to a second frequency, higher than the first frequency.
In a further aspect of the invention, the modulating signal is a baseband information signal.
In a further aspect of the invention, the phase modulation device comprises a baseband modulation circuit having an input port for input of a first baseband information signal and an output port for outputting a second baseband information signal which is conditioned for one of several types of frequency or phase modulation and a voltage controlled phase shifter having an input port for injection of a first signal having a first instantaneous phase and an input port for injection of the second conditioned baseband information signal and an output port for outputting a second signal having a second instantaneous phase which is proportional to the second conditioned baseband information signal and a first frequency/phase multiplier operatively connected to the voltage controlled phase shifter for receiving the second signal from the voltage controlled phase shifter and for expanding the instantaneous phase of the second signal to a value greater than 360 degrees.
In further aspects of the invention: the phase shifter preferably operates at microwave frequencies, the multiplier is formed with a field effect transistor, the multiplier has an integral multiplication factor; the voltage controlled phase shifter comprises a quadrature coupler having a pair of reflection ports and each of the reflection ports is terminated by equal reactive terminations; the reactive terminations comprise reverse biased varactor diodes, preferably abrupt or hyperabrupt varactor diodes with grounded series inductive reactances; to achieve higher multiplication factors, a second frequency/phase multiplier may be operatively connected to the first frequency/phase multiplier for receiving the second signal and for translating the second signal to a third signal having a third frequency, the third frequency being higher than the second frequency, the third signal preferably being a multiple of the second frequency; to achieve greater instantaneous phase expansion and/or lower phase error, a second frequency/phase multiplier may be operatively connected to the first frequency/phase multiplier for receiving the second signal and for further expanding the instantaneous phase of the second signal in a third signal having a third frequency, the third signal instantaneous phase being higher than the second signal and preferably greater than 360 degrees; the baseband modulation circuit accepts a baseband information signal, is comprised of a digital lookup table containing signal conditioning information, has a digital to analog converter, and outputs a conditioned baseband information signal for injection into the phase shifter.
In a still further aspect of the invention, the first frequency is translated in a FET, having a gate bias, the first signal has a signal level, and the gate bias and subharmonic input signal level are selected such that the FET has unconditional stability at all subharmonics of the second signal.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.